Why can't the server get the client MAC address, like the client IP?

Question

From what I know, all the MAC addresses that the packet gone through are in the packet. This is because that each packet that goes in a certain path, should also be returned

Answer 1

You have the relationship between the IP stack and the physical stack reversed. The MAC address is in the part that wraps the IP information, not the other way around. So when I send something from my computer, say, to yours, the physical segment in my LAN (Ethernet in this case) wraps around the IP stuff and includes my MAC. The router then extracts the IP information and passes that onward and upward (in this case again by Ethernet) to my DSL modem using its own MAC in the process. The DSL modem unwraps the IP stuff and sends that up the phone line using whatever protocol it uses (don't know and don't care at that point) out to ... well, you get the idea. Each physical link hop wraps and unwraps the IP information using whatever extra information the underlying physical layer involved attaches for local transmission.